1. Field of the Invention
This invention relates to a hydraulic shock absorber suitable for use in control of the attenuating force of a piston in a medium or high speed range of the speed of the piston and also in control of the attenuating force of the piston in a very low speed range of the speed of the piston.
2. Description of the Prior Art
In recent years, there is a tendency that a hydraulic shock absorber is formed such that, in order to improve, for example, the drivability and the stability in driving of a vehicle, it may perform not only control of attenuating force of a piston in a medium or high speed range of the speed of the piston but also control of the piston in a very low speed range of the speed of the piston.
An exemplary one of conventional hydraulic shock absorbers is disclosed, for example, in Japanese Utility Model Laid-Open No. 60-101242. The conventional hydraulic shock absorber is shown in FIG. 11. Referring to FIG. 11, the conventional hydraulic shock absorber includes a cylinder 1, a piston member 2 disposed in the cylinder 1 to define an upper or first oil chamber A and a lower or second oil chamber B in the cylinder 1 and constructed to communicate the oil chambers A and B with each other, and an expansion side attenuating valve 3 mounted on the piston member 2.
The attenuating valve 3 includes a leaf valve 3adisposed on the upstream side for generating the attenuating force in the very low speed range of the speed of the piston, and another leaf valve 3b disposed on the downstream side for generating an attenuating force in the medium or high speed range of the speed of the piston. Each of the leaf valves 3a and 3b is secured at an inner peripheral portion thereof and constructed to be deflected at an outer peripheral portion thereof to generate a predetermined attenuating force.
The leaf valve 3a is disposed such that it closes an opening at a lower end of an inner port 20a perforated in a piston body 20 which constitute the piston member 2 while the other leaf valve 3b is disposed such that it covers over the leaf valve 3a from below. A spring sheet 4 is disposed on a back or lower face of the leaf valve 3a, and it is normally acted upon by urging force of a spring 5 from below.
With the hydraulic shock absorber, when the piston member 2 is making an expanding movement in which it moves up in the cylinder 1 and the piston speed is in the very low speed range, an outer peripheral portion of the leaf valve 3a is deflected by a flow of hydraulic operating fluid from the upper oil chamber A which serves as a higher pressure side oil chamber, whereupon a predetermined attenuating force is generated. On the other hand, in the medium or high speed range of the piston speed, an outer peripheral portion of the leaf valve 3b is deflected, in addition to such deflection of the leaf valve 3a as described above, by a flow of hydraulic operating fluid from the oil chamber A to push down the spring sheet 4 overcoming the urging force of the spring 5, whereupon a predetermined attenuating force is generated.
Incidentally, the piston member 2 of the conventional hydraulic shock absorber is constructed such that hydraulic operating fluid coming around an outer peripheral portion of the leaf valve 3a is introduced into an outer port 20b perforated at a location of the piston body 20 radially outwardly of the inner port 20a by way of a sub port 20c.
The conventional hydraulic shock absorber. however, is disadvantageous in that the life of the leaf valve 3a is short.
In particular, the conventional hydraulic shock absorber carries out high attenuating force control when the leaf valve 3b is deflected. Thereupon, however, an outer peripheral portion of the leaf valve 3a is deflected simultaneously over a greater extent, and accordingly, the leaf valve 3a is deflected every time an attenuating force is generated by the hydraulic shock absorber. Since the leaf valve 3a is secured at an inner peripheral portion thereof, it is likely to be damaged of broken due to such repetitive deflections, which will decrease the life of the leaf valve 3a.